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We’ll start with some striking images taken by glaciologists around the world — these are photos of glaciers taken from the same spot at different times, and they provide us with some fairly shocking observations on just how much glaciers can change and have done so recently.

Muir Glacier in Alaska, as seen in 1941 and 2004. In the 2004 photograph, the glacier has melted almost completely.
Muir Glacier in Alaska, as seen in 1941 and 2004
Credit: Photo courtesy of William Field (1941) and Bruce Molnia (2004) and the National Snow and Ice Data Center, University of Colorado, Boulder.

For another Alaskan example, we turn to satellite imagery of Columbia Glacier, which obviously does not reach back into time as far, but nevertheless, there are some dramatic changes evident here:

Satellite image of Columbia Glacier, Alaska 1986
Aerial view of the Columbia Glacier, Alaska, 1986

The above scene is from 1986, and at this point in time, the end of the glacier (terminus) is located down near the bottom of the image. Below, we jump forward in time to 2011:

Satellite image of Columbia Glacier, Alaska 2011
Aerial view of the Columbia Glacier, Alaska, 2011
Credit: Modified from NASA’s Earth Observatory to show terminus locations

As you can see, the terminus here has retreated by about 15 km in just 25 years — a very impressive rate.

Four side-by-side images of Grinnell Glacier, Mt. Gould (1938, 1981, 1998, 2005) each more melted than the previous
Near disappearance of the famous Grinnell Glacier in Glacier National Park from 1938 to 2005

Grinnell Glacier, in Glacier National Park, Montana, as seen from the same vantage point over a 67 year period. The glaciers in this famous national park are all in such rapid retreat that the park may need a new name in a few decades.

Glaciers in the Alps are shrinking too. Check out SwissEduc Glaciers online to see one good example — at the bottom of the page is a comparison that flips back and forth from the past to the present as you move your mouse over the image.

The same story as seen in Alaska, Montana and the Alps holds for glaciers in more tropical settings, as can be seen from the images of Qori Kalas glacier in the Andes Mountains of Peru, below.

Two side-by-side photographic images of Qori Kalas Glacier Andes Mountains, Peru, 1978 and 2004 showing how much it has melted
Retreat of the Qori Kalas glacier in the Andes Mountains of Peru 1978 to 2004
Credit: Lonnie Thompson and the National Snow and Ice Data Center, University of Colorado, Boulder.

Studies of glaciers around the world show that an overwhelming majority are losing mass over time. In most cases, this loss of mass is reflected in the glaciers' retreating, so the length of the glacier becomes smaller. The figure below shows a selection of data from glaciers around the world, documenting this pattern of retreat.

Graph with examples of glacier length records from different parts of the world, see text below
This figure shows examples of glacier length records from different parts of the world. Data points are scarce before 1900; after 1900 a considerable number of records have an annual resolution. Data from Oerlemans, 2005, Science, v. 308, p. 675.
Credit: Robert Rohde

As can be seen in the figure above, a few glaciers show a pattern of increasing length in recent times. This can be a bit misleading, however, since the thing that really matters is the mass of ice in a glacier — that is the best measure of whether glaciers are growing or shrinking. When a glacier is melting rapidly, some of the meltwater can sink to the base and lubricate the base, in which case the glacier front can surge forward at impressive speeds. This causes the length of the glacier to increase even though the mass of the glacier is decreasing, thus explaining the increased length of a few of these glaciers. But in general, the vast majority of glaciers on Earth are melting, and this melting began about the time that the temperature records indicate the beginning of warming, around the beginning of the 1900s.

If you combine the records of glacier length changes from around the world into one graph, we can get a pretty clear idea of what is happening.

Graph combining the records of glacier length changes from around the world, 1700-2000
Change in average length of all glaciers around the world
Credit: Figure adapted from Oerlemans, 2005

On the graph above, the y-axis plots the length of the glaciers relative to their length in 1950 — so this is a kind of length anomaly. A positive number means that on average, glaciers were longer than they were in 1950; negative numbers mean they were shorter. Here, we can see that beginning around 1850, glaciers around the world begin to shrink, and this trend continues to the present. The average glacier has retreated almost 2 kilometers in this time.

It is possible to estimate the magnitude and history of temperature change needed to produce this history of glacial retreat, and Oerlemans (2005) did this using a simple model; the results are shown below.

Graph of temperature anomaly (glaciers, multi-proxy and instrumental), 1600-2000, showing a steep increase to the right of the graph
Modeled temperature change required to cause glacial retreat compared to proxy and instrumental temperature data
Credit: D. Bice; data compiled by Oerlemans, 2005

The thick blue line here is the temperature history needed to produce the timing and magnitude of the glacial retreat history shown in the previous figure. For comparison, we also see the instrumental temperature record in red (Jones and Moburg, 2003) and the temperature reconstruction based on multiple climate proxies (Mann et al., 1999). Note the excellent match with the instrumental record in the last century.